Recently, the applicability of neurons to electronic devices has been vigorously studied as well as the medical study. An action potential is generated in a neuron which is in an active state. A change in the ion permeability of a neuron leads to changes in intra- and extracellular ion concentrations which are responsible for generation of an action potential. Therefore, if a potential change in association with a change in ion concentration around a neuron is measured, the activity of the neuron can be monitored.
The above-described potential measurement utilizing cell activity is conventionally conducted by placing an electrode of glass or metal (e.g., platinum) for measuring an extracellular potential around a cell with the aid of a micro-manipulator or the like. Alternatively, a similar electrode is inserted into a cell so as to measure the electrical activity of the cell. These conventional techniques have the following disadvantages: skill in electrode preparation is required; the electrode has high impedance and therefore the signal is susceptible to external noise; and cells or tissues are injured if an electrode is inserted into the cell. Therefore, conventional electrodes are not suitable for long-term monitoring.
To avoid such problems, the inventors have developed a multiple electrode including a plurality of micro-electrodes made of a conductive material provided on an insulating substrate, and a lead pattern, on which cells or tissue can be cultured (Japanese Laid-Open Publication No. 6-78889, and Japanese Laid-Open Publication No. 6-296595). With this multiple electrode, the activities of neurons can be monitored without injuring cells or tissue for a long period of time.
In the above-described multiple electrode, an uppermost surface of the electrode contacting cells is plated with porous platinum black using electrolysis (Japanese Laid-Open Publication No. 6-78889), or with gold using deposition (Japanese Laid-Open Publication No. 6-296595). In the case of the platinum black plating, although it is easy to adjust the impedance of the electrode to a practical level, i.e., about 50 kΩ or less, the strength of the electrode is low and therefore the recyclability of the electrode is low. In the case of gold formed by deposition, the strength is improved, but it is difficult to reduce the impedance to about 50 kΩ or less.